Device dies, which include integrated circuit devices such as transistors therein, are often bonded to other package components such as package substrates, interposers, and Printed Circuit Boards (PCBs). The bonding may be performed through solder bonding, direct metal bonding, or the like. Since the device dies and the respective underlying package components have different Coefficients of Thermal Expansion (CTEs), the bonding structures such as the solder regions and metal pads often suffer from high stresses. For example, the CTEs of package substrates are often significantly greater than the CTE of silicon. As a result, due to the heating and cooling processes in the bonding process, stresses are applied to the bonding structures. To solve this problem, underfill is dispensed between the device dies and the underlying package components to protect the bonding structures.
The dispensing of underfill is through capillary. For example, to dispense the underfill into the gap between a device die and a package substrate, the underfill is dispensed onto a surface of the package substrate, wherein the underfill contacts a side of the device die. The underfilling process includes a plurality of dispensing steps, with a waiting time following each of the dispensing steps since the capillary takes time. During the waiting time, the underfill moves into the gap through capillary. Each of the dispensing steps and the respective waiting time causes the underfill to move further in the gap through capillary, until the underfill reaches the opposite end of the gap.
The conventional dispensing method often results in a wide fillet of underfill on the side that the underfill is dispensed. On other sides of the device dies, the fillet is significantly narrower. This causes non-uniformity in the profile of the underfill. Since underfill applies stresses to the device die and the bonding structures between the device die and the package substrate, the non-uniformity in the profile of the underfill causes the stress applied on different parts of the device dies and the bonding structures to be non-uniform. Furthermore, the fillet width may be different from package to package, and it is hard to determine and compensate for the stresses. Moreover, the wide fillet of underfill may restrict further processes, such as lid attachment or Package on Package (POP) bonding, which processes occur in the regions near the device die.